Integrated laccase delignification with improved lignocellulose recalcitrance for enhancing enzymatic saccharification of ensiled rice straw

The recalcitrant of straw biomass due to its severe lignification hinders its efficient use as a biofuel of feedstock. Therefore, this study was aimed to develop an integrated approach for treatment of rice straw to improve enzymatic saccharification and increase bioethanol production. We evaluated the effect of Lactobacillus plantarum (LP) and cellulases (CL), along with the ligninolytic enzyme laccase (Lac), on the quality of silage, enzymatic hydrolysis, and ethanol production from rice straw ensiled for 60 days. The rice straw was either ensiled without additive (control) or treated with five combinations of the additives, i.e., LP, CL, LP+CL, LP+Lac, or LP+CL+Lac (LPCL+Lac). The results indicated that adding laccase improved the fermentation and silage preservation quality of rice straw biomass by increasing lactic acid production and decreasing pH (<4) (P < 0.05). It also maintained high water-soluble carbohydrate and mono- and disaccharides. The LPCL+Lac treatment managed to reduce the lignin content of rice straw up to 3.82%. Fourier transform infrared spectroscopic analysis and scanning electron microscopic images confirmed that laccase degraded lignin and altered the physico-chemical structure of rice straw. The delignification and reducing the recalcitrance of lignocellulose of rice straw biomass with integrated laccase pretreatment (LPCL+Lac) resulted in release of high content of reducing sugar (52.42 mg/mL), and produced the highest quantity of ethanol (24.45 mg/mL). These results demonstrate that laccase effectively degraded lignin content of rice straw, and its integrated use with LP and CL provides a feasible approach for improving the preservation and bioconversion efficiency of highly lignified and recalcitrant straw biomass.